The present invention is related generally to data communication systems and, in particular, to free space optical data communication networks.
Traditional telecommunication systems that connect two or more sites with physical wire or cable are generally limited to relatively low-speed, low-capacity applications. In order to address these limitations, recently developed systems utilize optical fibers. Yet, fibers still require a physical cable connection. To remove this limitation, systems utilizing the free space transmission of one or more light beams modulated with data have been developed. Systems using such beams greatly improve data speed and capacity rates, up to 10 Gigabytes per second, over traditional wire-based systems and, at the same time, avoid the traditional communication system infrastructure cost of laying fiber cable to physically connect one site in the system to another. Instead of cables, free space optical communications systems consist, in part, of at least one transmit telescope and at least one receive telescope for sending and receiving information, respectfully, between two or more communications sites. Each of these telescopes contains optics, consisting of at least a primary mirror and a secondary mirror or a lens. The transmit telescope uses its optics to transmit the light beam to the receive telescope. The receive telescope uses its optics to focus the incoming light beam onto the focal plane of the telescope. Generally, each telescope is attached to a communications network or other source/destination of information. In operation, the transmit telescope receives information from its respective network via cable or wireless transmission, and then transmits a light beam modulated with this information to one or more destination receive telescopes. Each receive telescope then relays data to its intended destination in its respective network via a cable or wireless transmission.
The aforementioned free space communications systems would, therefore, appear to have the benefits of reducing costs associated with installing and maintaining physical hard-wired portions of networks while, at the same time, increasing transmission capacity. However, free space optical communications may be hampered by a variety of factors. For example, since the transmit and receive telescopes may be located a great distance from each other, initial alignment of the telescopes, to insure that the transmitted light beam is incident upon the focal plane of the receive telescope, may be difficult to achieve. Additionally, even if initially aligned, misalignment of the transmit and receive telescopes may result from any displacement of the light beam during transmission or any movement of either the transmit or receive telescopes or their respective physical mounting platforms. As a result of such misalignment, the transmitted light beam may not be incident upon the focal plane of the receiving telescope, or may only be partially incident thereupon, leading to a loss or degradation of communications connectivity.
Another problem with free space optical communications results from the variation in atmospheric conditions. Specifically, since conditions like fog or snow can interfere with the transmitted light beam in such systems, the transmit telescope must produce a light beam with power sufficient to maintain communications connectivity in such variable conditions. In the absence of such signal-degrading conditions, however, the power of the received light beam may overload the electronics of the receive telescope. While the power to the laser or laser amplifier can be reduced to compensate, this may mean operating the devices at gains where they do not operate efficiently. It would be highly desirable, therefore, to operate devices at a fixed power level sufficient for the worst expected condition and then attenuate the beam when necessary. A means for providing this attenuation is needed.
The aforementioned problems related to the receive telescope receiving from the transmit telescope a light beam having excessive power during communications operations are essentially eliminated with the present invention. In accordance with the present invention, the transmitted light beam and the receive optical fiber are adjusted with respect to each other when a specified power threshold is exceeded so that they intersect not at a point of focus, but at a point of divergence of the transmitted beam. At such a point, the cross-sectional area of the transmitted beam is larger and the received power per unit area is lower, thereby reducing the input power to the receive optical fiber.
In accordance with a first embodiment of the present invention, the divergence of the transmitted light beam is varied so as to effectively increase the cross-sectional area of the beam that is incident upon the receive telescope. This divergence of the transmitted beam is accomplished by moving the optical fiber, located at the focal plane of the primary mirror of the transmit telescope, to a point in front of the focal plane in the z-direction along the longitudinal axis of transmit telescope. The resulting beam produced by the transmitted telescope increases the cross-sectional area of the transmitted beam at the receive telescope. This larger cross section correspondingly results in a lower received power per unit area of the cross section. Therefore, any point on the focal plane of the receive telescope will be exposed to a lower power level per unit area and the receive optical fiber, at the focal plane of the receive telescope, will also receive a signal with reduced power. A similar reduction in power level per unit area may be achieved by moving the transmit optical fiber to a point to the rear of the focal plane in the z-direction along the longitudinal axis of the transmit telescope. Such a movement will result in a converging transmitted beam, which is more susceptible to atmospheric disturbances due to the resulting smaller beam cross section. However, the use of adaptive optics or other corrective measures will compensate for any signal interference resulting from such disturbances. By monitoring signal power at the receive telescope and providing feedback to the transmit telescope, the divergence of the transmitted beam is varied until the power level at the focal plane of the receive telescope is reduced to or below a specified threshold. If weather conditions deteriorate from optimum conditions, the transmit optical fiber can be moved back to a point closer to the focal plane along the z-axis of the transmit telescope""s primary mirror such that the transmitted beam achieves greater focus and, hence, greater power per unit cross sectional area.
Alternatively, in a second embodiment of the present invention, instead of moving the optical fiber at the transmit telescope, the optical fiber located at the focal plane of the primary mirror of the receive telescope may be moved to a point to the rear of the focal plane of the receive telescope in the z-direction along the longitudinal axis of the receive telescope. Alternatively, to achieve a similarly reduced signal power, the receive optical fiber may be moved to a point in front of the focal plane along the longitudinal axis of the receive telescope. Such a movement would not result in degraded received signal quality as may occur in the case of moving the transmit fiber. Since the maximum possible received power is achieved at the focal point within the focal plane, any movement of the receive optical fiber away from the focal point along the z-direction will result in a reduction of the received power. Once again, by monitoring received signal power, and adjusting the position of the received optical fiber, received power can be either increased or decreased as necessary.